I finally finished it last night. He put it on over his pyjamas first thing and has told me proudly that he is wearing it next time he goes to childcare.
I used the kids knits book for the basic pattern but I knitted it at 22st/10cm with a size 2/3 width and 4/5 length. The sleeves I measured against him rather than knitting to a pattern.
I made the sides of the neck match exactly by knitting one side right handed and the other left handed, this meant that both sides have k2tog for the decreases.
Hopefully it will last a year…